The present invention relates to apparatus for coating a moving web of paper or the like, and more particularly to new and improved coating apparatus that is capable of conversion selectively in a simple and highly effective manner to enable it to perform one or more different treatment operations on a moving web.
It is well known that the two sides of a paper web can be effectively coated by causing the web to run downwardly through a dam containing coating material and disposed immediately upstream of the nip between a thin, flexible coating blade and a support roller rotating in the direction of movement of the web, as disclosed in U.S. Pat. No. 3,489,592. In order to prevent so-called "orange peel" patterning, the coated web, upon leaving the nip is caused to be deflected at a suitable angle to the tangent to the support roller through the point of contact. Also, double sided web coating with different coating compositions can be performed by applying a second coating material to the surface of the support roller, as taught in U.S. Pat. No. 3,899,615.
Such apparatus can also be used for surface sizing of a web by supplying a starch solution to the dam through which the web passes. Since the pressure zone is extremely short in the direction of movement of the web because of the thinness of the coating blade (e.g. 0.3 mm), the impregnation of the web is not deep.
This is often a great advantage if the only aim is high surface strength and effective binding of the fibers on the surface of the paper. Since the binder is confined to the surface of the paper, less binder is required for a specific surface strength. Also, a higher concentration of treating solution can be used, leaving relatively little water to dry off, so that less energy is required for drying the web subsequently.
It is also old to improve the surface strength of a paper web surface sizing in a tub sizing machine comprising two rotating rollers forced together under pressure, at least one of the rollers usually being covered with rubber. In practice, the rollers may be disposed horizontally or diagonally in relation to each other and the paper web passes through a nip between them and through coating material contained in a dam formed by the two sides of the web and the adjacent roller surfaces immediately above the nip.
In surface sizing in a tub sizing machine, a relatively long pressure zone is produced in the direction of movement of the web. Accordingly, the treating liquid, starch solution for example, is first pressed inwardly towards the center of the web and is thereafter absorbed to a certain extent as the web gradually passes through the zone.
Good impregnation of the binder is essential if satisfactory inner strength is to be achieved with certain qualities of paper and cardboard. This may be of importance in the manufacture of cardboard, for example, which is to be folded and thus subjected to stress in the center of the sheet. Insufficient internal binding may result in the paper splitting. Good impregnation is also desired in the manufacture of paper and cardboard which is to be very stiff, this being achieved by causing the treating starch to penetrate to the center of the paper.
Heretofore, a manufacturer desiring to produce both products having high surface strength and products such as cardboard requiring good impregnation of a binder for satisfactory inner strength was required to have in his plant separate machines of the kinds described above. While such machines are effective in their respective fields, this practice is wasteful of space, expensive, and it may be inefficient if not all of the machines can be utilized simultaneously.